| Issue |
131444
|
| Summary |
[InstCombine] Infer exact for lshr by cttz
|
| Labels |
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
nikic
|
From https://github.com/rust-lang/rust/issues/138497:
```llvm
define i8 @src(i8 %x) {
%cttz = call i8 @llvm.cttz.i8(i8 %x, i1 false)
%lshr = lshr i8 %x, %cttz
ret i8 %lshr
}
define i8 @tgt(i8 %x) {
%cttz = call i8 @llvm.cttz.i8(i8 %x, i1 false)
%lshr = lshr exact i8 %x, %cttz
ret i8 %lshr
}
```
https://alive2.llvm.org/ce/z/KFemjy
This then allows folding `icmp eq %lshr, 0`.
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